Another possiblity! Depbo the addon file called "Flags", and choose the flag you want.
For example, you want to insert a US flag. so you depbo (using the program called Depbo ;D) "Flags", you pick the one called "USA.jpg", you put it inside your mission folder, then type in the init line 'this setflagtexture "USA.jpg"'...
There are other ways to put a flag on a pole, but i've always used this way of doing, and i will continue doing it like that
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Hoped i help u